This invention relates to primer paints and, particularly to a method of imparting corrosion-inhibition to metal primers.
Corrosion is an electrochemical phenomenon in which a metal in its elementary form is converted to a compound, usually an oxide. The presence of materials such as chlorides, sulfates, and sulfides, even in very small amounts, can greatly accelerate corrosion of a metal substrate. Since metallic structures can be exposed to conditions ranging from total immersion in fresh or salt water to atmospheres high in humidity or chemical impurities, a protective coating composition, i.e., a primer, specially formulated for its ability to inhibit corrosion of the metal substrate is generally applied. Since no primer will protect against corrosion equally well under all circumstances, the choice of pigment and vehicle which make up the primer depends primarily on the proposed application.
A variety of pigments are known in the art for their ability to inhibit corrosion of metal substrates when applied to the substrate in a primer. The most commonly used corrosion-inhibitive pigments include zinc yellow (4ZnO.4CrO.sub.3.K.sub.2 O.3H.sub.2 O), basic zinc chromate (5ZnO.CrO.sub.3.4H.sub.2 O), strontium chromate (SrCrO.sub.4), zinc oxide (ZnO), red lead (Pb.sub.3 O.sub.4), basic lead chromate (PbO.PbCrO.sub.4), basic lead silicochromate (PbO.PbCrO.sub.4 + SiO.sub.2) and zinc dust (Zn).
Although each of these corrosion inhibitive pigments has been found to have some value in use, in some cases there have been difficulties in their production and large scale commercialization. None of the pigments disclosed in the prior art has been completely satisfactory in all respects, although zinc yellow is generally considered the most versatile and is the most widely used. Other problems associated with many of the known pigments include high materials cost and manufacturing costs, the irritating and toxic effects of certain of the chemicals comprising the pigments, the unstable nature of the pigments in film forming media, the lack of color stability upon exposure to sun and weathering, and the reactivity of the pigment with the film forming material to effect breakdown of the film.
This invention provides for an economic method for imparting corrosion-inhibition to primers by incorporating therein an inexpensive inorganic pigment which generally exhibits higher corrosion-inhibition than zinc yellow and can be manufactured on a commercial scale in a substantially pollution free manner.